[Bug gold/22791] Don't error when a -pie executable uses a shared function without PIC/PIE relocation

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=22791

--- Comment #14 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Cary Coutant from comment #13)
> (In reply to H.J. Lu from comment #12)
> > Since there is no need to prepare for PLT branch on x86-64, we can treat
> > PC32 relocation with branch as PLT32 relocation.  I posted a patch for ld:
> > 
> > https://sourceware.org/ml/binutils/2018-02/msg00065.html
> 
> What do you mean by "prepare for PLT branch"?

On i386, there are 2 types of PLTs, PIC and non-PIC.  PIE and shared objects
must use PIC PLT.  To use PIC PLT, you need to load _GLOBAL_OFFSET_TABLE_
into EBX first.  There is no need for that on x86-64.

> It looks like your patch examines the opcode(s) preceding the relocated
> value to determine whether it's a branch or not. How is this safe? It seems
> fragile to me. In addition, it requires the linker to read section contents
> while scanning relocations, which is not normally necessary, and will slow
> the linker down significantly -- we normally don't need to read most
> sections' contents until it's time to apply relocations.
> 
> If your argument is that you can always treat PC32 relocations on branches
> as if they were PLT32 relocations, why not just have the compiler emit PLT32
> relocations in the first place?

Yes, this patch does that:

https://sourceware.org/ml/binutils/2018-02/msg00074.html

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